The present invention relates to a synthetic quartz glass member for optics having excellent optical transmittance, homogeneity, and stability with respect to the irradiation of a high intensity ultraviolet radiation, in particular, to those of KrF and ArF excimer lasers. More specifically, the synthetic quartz glass members according to the present invention relate to those favorably applicable to lenses of aligners and other optical components for use in lithography using excimer lasers for producing semiconductor chips, as well as for prisms used in narrowing bands of excimer lasers, other general use optical members utilized in KrF and ArF excimer laser radiations, lenses, beam splitters, prisms, etc.
Recently, practical LSIs are being produced by KrF excimer laser lithography, and developments targeted to the realization of ArF excimer laser lithography for the next generation exposure technology are in progress. Furthermore, improvements are also made on KrF lithography, such that the performance of patterning finer circuit patterns or the processing capability (throughput) per hour can be improved. More specifically, fine circuit patterns are now produced by employing improved exposure methods such as modified illumination method and phase shift masks, and this fundamentally requires an aligner equipped with a lens made from a material of higher performance. That is, a lens material having higher homogeneity and low birefringence is strongly demanded for accurate focusing. Furthermore, in order to improve the throughput, the quantity of exposure light for use in patterning is being increased by elevating the repetition frequency or the energy of the excimer laser. In such a case, the lens material tends to be optically damaged due to the increased quantity of light transmitted through the lens material of the aligner. At any rate, a quartz glass for use as the lens material with higher quality is required every year with the increase in performance of the aligner. The means now being taken based on prior art for improving homogeneity or for increasing laser resistance is found insufficient.
The technique for obtaining a quartz glass of high homogeneity has been studied in various fields, and many patents applications have been made thereon. In general, the homogeneity of an optical quartz glass signifies the uniformity in refractive index. Generally, a retardation in phase is measured by an interferometer using a Hexe2x80x94Ne laser emitting a radiation 632.8 nm in wavelength to calculate the distribution in refractive indices. A highly uniform quartz glass lens material is necessary because an extremely high imaging performance is required on a lens for use in lithography. The refractive index changes depending on the impurities incorporated in the quartz glass, e.g., chlorine or hydroxyl groups, etc., or on the fictive temperature. Thus, if the quartz glass contains impurities distributed therein, the distribution of refractive indices changes depending thereon. For instance, a portion containing SiOH groups in a larger quantity yields a relatively low refractive index. The fictive temperature changes depending on the cooling rate in thermally treating quartz glass, and a distribution in refractive index is also formed due to the generation of a distribution in fictive temperature within the quartz glass. In JP-A-Hei3-88743 (the term xe2x80x9cJP-Axe2x80x9d as referred herein signifies xe2x80x9can unexamined published Japanese patent applicationxe2x80x9d) is disclosed a method for producing a glass of high homogeneity by positively combining the factors above inducing fluctuation in refractive index. Furthermore, the same published patent application describes a method of imparting high laser resistance to the quartz glass by thermally treating the glass in a hydrogen-containing atmosphere to thereby enable the doping of hydrogen molecules. That is, by doping glass with hydrogen, the technique enables obtaining a glass of high homogeneity while maintaining high resistance against an excimer laser radiation. Accordingly, since a high resistance against laser radiation can be imparted by applying hydrogen doping to a highly homogeneous quartz glass obtained by controlling the distribution of impurities such as SiOH and chlorine, it has been believed that such a material could be favorably used as a lens material for use in excimer laser lithography. In fact, however, there has been found practical problems such that (1) an even higher resistance against laser radiation is required recently in view of increased repetition times and higher energy used in laser, and (2) homogeneous doping of hydrogen molecules is difficult in view of the recent demand on larger materials having even higher NA. Thus, a material sufficiently satisfying the requirements cannot be obtained by a method described in the published patent application above. In particular, taking the diffusion coefficient of hydrogen molecules into consideration, it is extremely difficult to realize a large material uniformly doped with hydrogen molecules by performing heat treatment in an atmosphere containing hydrogen. In practice, there are problems as such that, by calculation, such a process requires an extremely long time of treatment or a hydrogen addition treatment at a considerably high temperature. Furthermore, from the studies to present, it has been found that hydrogen molecules also influence the distribution in refractive indices, and that uniformly doped hydrogen molecules make an important factor.
Homogenous hydrogen doping is important in obtaining a highly homogeneous quartz glass yet having high resistance against laser radiation, however, particularly in case of a large material, a long time duration is necessary to sufficiently diffuse hydrogen deep into the inside of the material. This results in a serious problem of impairing the productivity. Applying hydrogen doping at higher temperatures at which the diffusion coefficient of hydrogen becomes higher can shorten the time necessary for diffusing hydrogen. However, a quartz glass exposed to hydrogen at high temperatures tend to newly generate defects, and this is known to impair the resistance against laser radiation of the quartz glass. JP-A-Hei6-166522 describes the resistance against the irradiation of an ArF excimer laser of a hydrogen-doped material obtained by thermally treating synthetic quartz glass member trial produced materials at various temperatures under a gaseous hydrogen atmosphere. It shows that the material subjected to a heat treatment at high temperatures exceeding 600xc2x0 C. yields an extremely impaired resistance against laser radiation. This is attributed to the fact that reductive defects generate inside the quartz glass due to the hydrogen molecules introduced at higher temperatures, which induce absorption in the ultraviolet region in case an excimer laser is irradiated thereto. The absorption thus induced is ascribed to paramagnetic defects generally denoted as Exe2x80x2 centers, and yields an absorption band having its center wavelength in the range of from 210 to 215 nm. Thus, in order to suppress the generation of Exe2x80x2 centers, i.e., to suppress the generation of induced absorption, the published patent application above describes the application of a heat treatment at a temperature not higher than 600xc2x0 C. As described above, however, the diffusion coefficient for hydrogen decreases at lower temperatures, and this increases the process time in case of processing a large material as to impair the productivity. Accordingly, in order to introduce more hydrogen molecules into the quartz glass, the published patent above describes a treatment carried out under a high pressure of 1 atm or higher, preferably, 50 atm or higher. However, in general, the distribution of hydrogen molecule concentration for a quartz glass thus obtained yields, in accordance with the principle of a diffusion phenomenon, higher concentration values at the peripheral portions as compared with those at the central portion. In case of a treatment performed at higher pressures, in particular, it results in a larger difference as to greatly affect the homogeneity (in the distribution of refractive indices).
In JP-A-2000-95535 is disclosed a method of hydrogen doping under a high pressure, such that the formation of hydrogen concentration distribution is suppressed in such a manner that the homogeneity would not be affected. According to the published patent application, the method comprises doping hydrogen molecules under a pressure of 10 atm or higher, preferably, 50 atm or higher (second step), and performing heat treatment (third step), thereby selectively degassing the peripheral portion having a high hydrogen molecule concentration as to obtain a quartz glass material having a uniform hydrogen concentration distribution. However, the method according to this published patent application requires applying heat treatment in two stages, i.e., a heat treatment for doping hydrogen (second step) and a subsequent heat treatment under an atmosphere free from hydrogen (third step). Furthermore, if the latter heat treatment should be performed at too high a temperature, it is known that the resistance against laser radiation is impaired by the newly formed defects. Accordingly, in case of selectively degassing the peripheral portion having high hydrogen concentration alone, the process should be performed at a relatively low temperature, and this requires a heat treatment performed for a long duration of time. Furthermore, the concentration and the distribution of hydrogen greatly fluctuate depending on the size of the molded body or on the conditions of the second step, and it was found quite difficult to obtain a hydrogen concentration with a flat distribution by selectively and properly degassing the peripheral portion alone in a simple heat treatment that is performed in the third step. As described above, an additional heat treatment performed after doping hydrogen was not so preferable from the viewpoints of resistance against laser radiation and from productivity.
In the light of the disadvantages of prior art technology, an object of the present invention is to provide a method for producing a synthetic quartz glass member for excimer lasers in which, while suppressing the generation of reductive defects which impairs the resistance against laser radiations, incorporating a sufficient amount of hydrogen molecules capable of achieving a high resistance against laser radiation, yet uniformly incorporating the hydrogen molecules to realize a flat distribution in refractive indices attributed to the distribution in the density of hydrogen molecules. It is also an object of the present invention to provide a synthetic quartz glass member for excimer lasers obtained by the production method above, which yields high resistance against laser radiations and homogeneity.
The above objects are achieved by one of the constitutions described in (1) to (10) below.
(1) A method for producing a synthetic quartz glass member for excimer lasers, which, in a method for producing a synthetic quartz glass member for excimer laser optics comprising a step of incorporating hydrogen molecules into a synthetic quartz glass body by heat treating the synthetic quartz glass body at a temperature of 600xc2x0 C. or lower under an atmosphere in a pressure range of 1 atm or higher but lower than 150 atm and containing hydrogen, said method comprises varying the pressure of the gas containing hydrogen either continuously or stepwise in at least a part of the heat treatment.
(2) A method for producing a synthetic quartz glass member for excimer lasers as described in (1) above, wherein the pressure of the gas containing hydrogen is decreased.
(3) A method for producing a synthetic quartz glass member for excimer lasers as described in (2) above, wherein the synthetic quartz glass body is heat treated under an atmosphere containing hydrogen for a first predetermined time duration under a pressure of first setting, and is further heat treated for a second predetermined time duration under a pressure of second setting that is lower than said pressure of first setting.
(4) A method for producing a synthetic quartz glass member for excimer lasers as described in one of (1) to (3) above, wherein the atmosphere containing hydrogen consists of 100% gaseous hydrogen or a mixed gas containing hydrogen together with nitrogen, argon, or helium.
(5) A method for producing a synthetic quartz glass member for excimer lasers as described in one of (1) to (4) above, wherein the synthetic quartz glass body in which hydrogen is incorporated is prepared by either a direct flame hydrolysis method or an indirect flame hydrolysis method.
(6) A synthetic quartz glass member for excimer lasers produced by a method for producing a synthetic quartz glass member for excimer lasers as described in one of claims 1 to 5, in which hydrogen is uniformly incorporated therein.
(7) A synthetic quartz glass member for excimer lasers as described in (7) above, wherein the average density of the hydrogen molecules is 1xc3x971018 molecules/cm3 or higher, and the fluctuation in the density of hydrogen molecules is within a range of xc2x11.2xc3x971018 molecules/cm3.
(8) A synthetic quartz glass member for excimer lasers as described in (6) or (7) above, wherein, in case an ArF excimer laser radiation is irradiated thereto for a repetition of 2xc3x97105 pulses with an energy density per pulse of 2 mJ/cm2 at a frequency of 200 Hz, the member yields an induced absorption under an irradiation wavelength of 215 nm of 0.003 or lower as expressed by absorbance for a thickness of 1 cm.
(9) A synthetic quartz glass member for excimer lasers as described in (6) or (7) above, wherein, in case a KrF excimer laser radiation is irradiated thereto for a repetition of 2xc3x97105 pulses with an energy density per pulse of 100 mJ/cm2 at a frequency of 200 Hz, the member yields an induced absorption under an irradiation wavelength of 210 nm of 0.0075 or lower as expressed by absorbance for a thickness of 1 cm.
(10) A synthetic quartz glass member for excimer lasers as described in one of (7) to (9) above, wherein, under a radiation of 632.8 nm, it yields a homogeneity in refractive index of xc2x14xc3x9710xe2x88x926 (/cm) or lower and a birefringence of 2 nm/cm or lower.
The member obtained in accordance with the present invention is favorably employed in various types of apparatuses equipped with excimer lasers, particularly those for KrF or ArF excimer lasers; the member realizes a high resistance against laser radiation and homogeneity with high compatibility because hydrogen molecules are incorporated into the member at a high density and at a high homogeneity, and can be used stably for a long time as an optical member assembled in various types of apparatuses using the aforementioned excimer lasers. For instance, it can be suitably used as an optical member of an excimer laser stepper, a laser processing apparatus, a laser annealing apparatus, etc.